Optical transceiver with in-band management channel

ABSTRACT

A supervisory signal is superimposed onto a high-speed data stream so that the number of optical transceivers needed by an optical network is reduced. The supervisory signal is superimposed onto the high-speed data stream as an in-band modulation of the data stream. To improve signal-to-noise ratio of the in-band supervisory signal, the supervisory signal is first modulated to a higher frequency before it is superimposed onto the high-speed data stream.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention relate generally to opticalcommunication systems and, more particularly, to an optical transceiverused in such systems.

2. Description of the Related Art

Optical networks are used extensively in telecommunications for voiceand other applications. As utilization of optical communication networksincreases, there is an ongoing effort to lower the per-bit cost of datatransport. Some components of optical communication networks becomeincreasingly expensive when designed for higher speed optical networks,such as 1 Gigabit Ethernet (1 GbE), 2.5 Gigabit SONET networks, andfaster networks. For this reason, the added cost of high-speedcomponents can partially negate the per-bit cost savings associated withupgrading an optical communications network to a higher bit rate.

One relatively expensive component of an optical communications networkis the optical transceiver, for example the small form-factor pluggable(SFP) transceiver. Optical transceivers are located at each node of anoptical network, and interface a network switch, router, or similardevice with a fiber optic networking cable. Optical transceivers arerequired for data signals and a separate optical transceiver is requiredfor a supervisory signal.

Each supervisory signal, also referred to as an optical supervisorychannel (OSC), is propagated together with data signals along an opticallink established between nodes of the network, and contains informationfor maintaining and monitoring the optical link, including input power,output power, node temperature, etc. In addition, the OSC may be usedfor remote upgrades of the software controlling network devicescontained in network nodes. Because the OSC transceiver at each networknode does not increase the data transport capacity of the network, eachOSC transceiver negatively impacts the per-bit cost of data transportfor the network. This is especially true for networks designed tomultiplex a relatively small number of data signals onto a singleoptical fiber, such as coarse wavelength-division multiplexing (CWDM)systems.

Accordingly, there is a need in the art for a low-cost data transportsolution for high-speed optical networks that does not require an OSCtransceiver at each node of the network.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a method and apparatus forhigh-speed, low-cost transport of data signals that eliminate the needfor dedicated OSC transceivers in an optical network.

In one embodiment of the invention, an optical transceiver for acommunications network comprises an optical-to-electrical assemblyconfigured to receive a first optical signal containing a first datasignal and a first supervisory signal and separate the first opticalsignal into the first data signal and the first supervisory signal, andan electrical-to-optical assembly configured to receive a second datasignal and a second supervisory signal, and generate a second opticalsignal containing the second data signal and the second supervisorysignal.

A method of transmitting a supervisory signal between a first and secondnode of an optical communication network, according to an embodiment ofthe invention, comprises the steps of receiving a supervisory signal,combining the supervisory signal with a data signal having a frequencyof at least 1 GHz, converting the combined signal to an optical signal,and transmitting the optical signal containing the supervisory signalfrom the first node to the second node.

A method of extracting a supervisory signal from a combined signalreceived from a node of an optical communication network, according toan embodiment of the invention, comprises the steps of receiving acombined signal having the supervisory signal and a data signal having afrequency of at least 1 GHz, extracting the data signal, and extractingthe supervisory signal.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 compares the amplitude of 1/f noise in an optical link to theamplitudes of different data streams that may be transmitted through theoptical link.

FIG. 2 schematically illustrates a supervisory signal, a modulatedsupervisory signal, a data signal, and a modulated data signal, that aregenerated according to an embodiment of the invention.

FIG. 3 schematically illustrates an optical transceiver configured tosuperimpose a modulated supervisory signal onto a high-speed data signaland separate a modulated supervisory signal from a high-speed datasignal, according to an embodiment of the invention.

FIG. 4 is a flow chart summarizing an operating sequence for the opticaltransceiver depicted in FIG. 3, according to an embodiment of theinvention.

FIG. 5 is a flow chart summarizing another operating sequence for theoptical transceiver depicted in FIG. 3, according to an embodiment ofthe invention.

For clarity, identical reference numbers have been used, whereapplicable, to designate identical elements that are common betweenfigures. It is contemplated that features of one embodiment may beincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

Embodiments of the invention contemplate a method and apparatus formultiplexing, or combining, a supervisory signal with a high-speed datastream to eliminate the need for an OSC transceiver at each node of anoptical network. The supervisory signal is incorporated into thehigh-speed data stream as an in-band modulation of the data stream. Thisis unlike a conventional supervisory signal, which is typicallytransmitted in a separate wavelength channel that has a wavelengthoutside the data band of the network. According to embodiments of theinvention, the supervisory signal is superimposed as a modulation on theexisting data stream, therefore a dedicated optical transceiver is notrequired to transmit or receive the OSC.

FIG. 1 compares the amplitude of 1/f noise in an optical link to theamplitudes of different data streams that may be transmitted through theoptical link. Curve 153 represents the amplitude of 1/f noise, alsoreferred to as “pink noise,” present in an optical link. As implied byits name, 1/f noise refers to a signal or process with a spectral powerdensity inversely proportional to a frequency, f, associated with thesignal or process. In the context of optical communication systems, frefers to the data frequency of optical signals transmitted via anoptical link. Curve 150 represents a high-speed data signal having adata frequency of 1 GHz. Similarly, curve 151 represents an opticalsignal having a data frequency of 1 MHz and curve 152 represents anoptical signal having a data frequency of approximately 10 kHz. Asshown, a 1 MHz signal transmitted simultaneously through an optical linkwith a high-speed data signal may have a small amplitude relative to the1 GHz signal and still be distinguishable from 1/f noise present in theoptical link. To with, a 1 MHz signal, represented by curve 151, mayhave an amplitude 151A that is a small fraction of amplitude 150A of ahigh-speed data signal, represented by curve 150. Although amplitude151A is a fraction of amplitude 150A, curve 151 has a favorablesignal-to-noise ratio. In contrast, a 10 kHz signal, represented bycurve 152, is obscured by 1/f noise, i.e., has a low signal-to-noiseratio, even when the 10 kHz signal has an amplitude 152A that isapproximately equal to amplitude 150A of the high-speed data signal.

According to embodiments of the invention, a two-layer modulation of alow frequency supervisory signal onto a high-speed data signal in theGHz regime allows the incorporation of the low frequency supervisorysignal into the high-speed data signal as an in-band subcarrier. Asdefined herein, a subcarrier is a separate, lower frequency signalmodulated into a higher frequency primary signal. The supervisory signalmay have a frequency as low as 1 kHz, and may first be modulated at anintermediate frequency before being incorporated into the high-speeddata signal as a subcarrier. The use of an intermediate modulationfrequency substantially improves the signal-to-noise ratio of thesubcarrier.

In one embodiment, the supervisory signal is a 9.6 kHz signal that ismodulated at an intermediate frequency of 1 MHz and then superimposed ona 1 GHz data signal, where the modulation depth of the 1 MHz signal isabout 5%. Modulation depth, as defined herein, is the ratio of theamplitude of a subcarrier to the amplitude of the primary signal onwhich the subcarrier is superimposed. For reasons commonly known in theart, the modulation depth of a subcarrier is preferably less than about10% in order to avoid adversely affecting the data contained in theprimary signal. In this embodiment, the 1 GHz data signal serves as theprimary signal and the supervisory signal modulated at 1 MHz serves asthe subcarrier. In an alternative embodiment, the 9.6 kHz supervisorysignal may be directly modulated onto the 1 GHz data signal, but this isless desirable because the signal-to-noise ratio for the supervisorysignal will be much lower.

FIG. 2 schematically illustrates four signals that are generated inaccordance with an embodiment of the invention. The four signals includea supervisory signal 210, a modulated supervisory signal 220, a datasignal 230, and a modulated data signal 240. Because the frequencies ofthese signals may vary by several orders of magnitude, the relativewavelengths of these signals are not shown to scale for clarity.Supervisory signal 210, modulated supervisory signal 220, data signal230, and modulated data signal 240 are depicted as square waves,although it is understood that each may be in a sinusoidal or otherwaveform.

Supervisory signal 210 is a low frequency signal, such as a 9.6 kHzRS232 signal, carrying the management data required to maintain anoptical link established between two nodes of an optical network.Supervisory signal 210 contains a series of bits 211, where each bit iseither a “1” or a “0.” In the example illustrated, bits 211A correspondto 1's and bit 211B corresponds to a 0.

Modulated supervisory signal 220 represents supervisory signal 210 afterbeing modulated at a substantially higher frequency, in this embodimenton the order of 1 MHz. Modulated supervisory signal 220 contains aseries of bits 221 that carries the identical low frequency signal asthe series of bits 211 of supervisory signal 210. In modulatedsupervisory signal 220, however, each bit 221A and 221B is modulated atthe 1 MHz frequency, as shown. This higher frequency modulation allowsthe information contained in supervisory signal 210 to be superimposedonto a high-speed data stream, i.e., a data stream having a frequency of1 GHz or above, without being obscured by pink noise. In addition,because the magnitude of pink noise is substantially lower in the MHzregime than the kHz regime, the modulation depth of modulatedsupervisory signal 220 may be maintained relatively low. This minimizesinterference between modulated supervisory signal 220 and modulated datasignal 240, thereby preventing modulated supervisory signal 220 fromadversely affecting the data contained in modulated data signal 240.

Data signal 230 represents a high-speed optical data stream carryinginformation to be transmitted between two nodes of an optical network.In the embodiment illustrated in FIG. 2, data signal 230 is a datastream having a frequency of 1 GHz or faster, such as a 1 Giga-bitEthernet (1 GbE) signal or a 2.5 Giga-bit SONET signal. As shown, datasignal 230 has an amplitude 232. Because amplitude 232 is approximatelyten times greater than amplitude 222, the data traffic contained in datasignal 230 will not be adversely affected when data signal 232 iscombined with modulated supervisory signal 220 to form modulated datasignal 240.

Modulated data signal 240 is a high-speed data signal corresponding todata signal 230 after the addition of modulated supervisory signal 220,which acts as a subcarrier having a frequency on the order of 1 MHz.Modulated supervisory signal 220 may be superimposed onto data signal230 via an optical transceiver to form modulated data signal 240 priorto transmission of modulated data signal 240 from a network node. Themodulation occurs when data signal 230 and modulated supervisory signal220 are converted to a single optical signal, i.e., modulated datasignal 240, by the optical transceiver. Thus, modulated data signal 240includes the management information from supervisory signal 210 inaddition to the information carried by data signal 230. Because theinformation from supervisory signal 210 is included in modulated datasignal 240 as an in-band modulation, an additional transceiver forsending and receiving the supervisory information is not necessary.

In one embodiment, modulated supervisory signal 220 has an amplitude 222that is between about 3% and 10% of amplitude 232 of data signal 230.Hence, the modulation depth of modulated supervisory signal 220 is alsobetween about 3% and 10%. The optimal modulation depth of modulatedsupervisory signal 220 is a function of the transmission distance ofmodulated data signal 240, among other factors. This is because there isa performance trade-off between having a lower and a higher modulationdepth for modulated supervisory signal 220. Lower modulation depthresults in a lower signal-to-noise ratio for modulated supervisorysignal 220, which is problematic for longer transmission distances.Higher modulation depth increases the signal-to-noise ratio formodulated supervisory signal 220, but may adversely affect the datacontained in modulated data signal 240. Based on the foregoing, anoptimal modulation depth for modulated supervisory signal 220 can bereadily calculated.

Reconfigurable networks are currently under development, wherein theoptical distance between two nodes of a network may change substantiallydepending on network utilization and other factors. For this reason,embodiments of the invention contemplate a modulated supervisory signal220 having an adjustable modulation depth. In one embodiment, themodulation depth of modulated supervisory signal 220 may be variedbetween about 3% and about 10%, depending on changes in the transmissiondistance of modulated data signal 240 when the optical network isreconfigured.

FIG. 3 schematically illustrates an optical transceiver 300 configuredto superimpose a modulated supervisory signal onto a high-speed datasignal and separate a modulated supervisory signal from a high-speeddata signal, according to an embodiment of the invention. In thisembodiment, optical transceiver 300 is an SFP transceiver located at anode in an optical communication network. Optical transceiver 300includes an optical-to-electrical assembly 310, an electrical-to-opticalassembly 320, and a supervisory channel module 330, and is configured tosend, receive, or otherwise process supervisory signal 210, modulatedsupervisory signal 220, data signal 230, and modulated data signal 240,which are described above in conjunction with FIG. 2.

Optical-to-electrical assembly 310 is configured to receive modulateddata signal 240 from an adjacent network node and convert modulated datasignal 240 into two separate signals: supervisory signal 210 and datasignal 230. Optical-to-electrical assembly 310 includes a receiveoptical subassembly (ROSA) 311, a trans-impedance amplifier 312, a limitamplifier 313, a bandpass filter 314, and an operational amplifier 315.ROSA 311 receives modulated data signal 240 from an adjacent networknode, converts modulated data signal 240 into a modulated current signal317, and transmits modulated current signal 317 to trans-impedanceamplifier 312. Trans-impedance amplifier 312, which is acurrent-to-voltage converter, coverts modulated current signal 317 tomodulated voltage signal 318. Modulated voltage signal 318 contains thesame information as modulated data signal 240, i.e., a high-speed datasignal with a two-layer modulation containing a lower frequencysupervisory signal. As shown, a portion of modulated voltage signal 318is directed to limit amplifier 313 and a portion is directed to bandpassfilter 314 and operational amplifier 315. Limit amplifier 313 extractsdata signal 230 from modulated voltage signal 318 for output to thenetwork node containing optical transceiver 300. Together, bandpassfilter 314 and operational amplifier 315 separate supervisory signal 210from modulated voltage signal 318. Supervisory signal 210 is transmittedto supervisory channel module 330 via receiving universal asynchronousreceiver/transmitter (UART) 334.

Electrical-to-optical assembly 320 is configured to receive data signal230 and supervisory signal 210, modulate supervisory signal 210 ontodata signal 230, and produce and transmit modulated data signal 240.Electrical-to-optical assembly 320 includes an input port 321, anoscillator 322, an operational amplifier 323, a laser driver 324, and atransmit optical subassembly (TOSA) 325. Input port 321 is configured toreceive supervisory signal 210 from supervisory channel module 330 viatransmitting UART 335. Oscillator 322 modulates supervisory signal 210to produce modulated supervisory signal 220. Operational amplifier 323couples input port 321 to laser driver 324 and adjusts the amplitude ofmodulated supervisory signal 220 higher or lower as required so thatmodulated supervisory signal 220 has a desired modulation depth whensuperimposed onto data signal 230. Laser driver 324 receives modulatedsupervisory signal 220 and data signal 230 from the network nodecontaining optical transceiver 300, superimposes these signals toproduce laser control signal 326, and transmits laser control signal 326to TOSA 325. TOSA 325 converts laser control signal 326 into modulateddata signal 240 and transmits modulated data signal 240 to an adjacentnetwork node.

Supervisory channel module 330 is configured to receive a supervisorysignal from the network node containing optical transceiver 300, convertthe supervisory signal to supervisory signal 210 and transmitsupervisory signal 210 to input port 321 of electrical-to-opticalassembly 320. Similarly, supervisory channel module 330 is alsoconfigured to receive supervisory signal 210 from optical-to-electricalassembly 310, convert supervisory signal 210 to an appropriate format,and transmit the reformatted supervisory signal to the network nodecontaining optical transceiver 300. Supervisory channel module 330includes a bi-directional data line 331, a field-programmable gate array(FPGA 332), EEPROM 333 for programming FPGA 332, a receiving UART 334,and a transmitting UART 335. Bi-directional data line 331 is a standardcomputer bus that links supervisory channel module 330 to the networknode containing optical transceiver 300. One protocol commonly used inthe art for interfacing a supervisory channel with an opticaltransceiver is I²C. FPGA 332 is configured to convert supervisory signal210 received via receiving UART 334 to an I²C or other protocol tointerface with the network node. Similarly, FPGA 332 is configured toconvert a supervisory signal received from the network node tosupervisory signal 210 for transmission to electrical-to-opticalassembly 320 via transmitting UART 335.

FIG. 4 is a flow chart summarizing an operating sequence 400 for opticaltransceiver 300, according to an embodiment of the invention. Operatingsequence 400 describes the operation of optical transceiver 300 includedin a network node when receiving a modulated data signal 240 from anadjacent network node.

In step 401, ROSA 311 receives modulated data signal 240 and convertsthe signal to modulated current signal 317.

In step 402, trans-impedance amplifier 312 coverts modulated currentsignal 317 to modulated voltage signal 318.

In step 403, limit amplifier 313 extracts data signal 230 from modulatedvoltage signal 318, transmitting data signal 230 as required to thenetwork node.

In step 404, bandpass filter 314 and operational amplifier 315 separatesupervisory signal 210 from modulated voltage signal 318 and transmitsupervisory signal 210 to supervisory channel module 330.

In step 405, supervisory channel module 330 receives supervisory signal210 and FPGA 332 converts the signal to an I²C protocol.

In step 406, supervisory channel module 330 transmits the I²C-formattedsupervisory signal to the network node.

FIG. 5 is a flow chart summarizing an operating sequence 500 for opticaltransceiver 300, according to an embodiment of the invention. Operatingsequence 500 describes the operation of optical transceiver 300 includedin a network node when receiving a data signal 230 and supervisorysignal 210 from the network node containing optical transceiver 300.

In step 501, supervisory channel module 330 receives an I²C-formattedsupervisory signal and FPGA 332 converts the signal to an RS232 protocolsignal, i.e., supervisory signal 210.

In step 502, electrical-to-optical assembly 320 receives supervisorysignal 210 via electrical input port 321 and oscillator 322 modulatesthe signal at 1 MHz to produce modulated supervisory signal 220.

In step 503, operational amplifier 323 adjusts the amplitude ofmodulated supervisory signal 220 to a desired modulation depth relativeto data signal 230.

In step 504, laser driver 324 superimposes modulated supervisory signal220 and data signal 230 to produce laser control signal 326.

In step 505, TOSA 325 receives laser control signal 326 and convertslaser control signal 326 into modulated data signal 240.

In step 506, modulated data signal 240 is transmitted to an adjacentnetwork node.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. An optical transceiver for a communications network, comprising: anoptical-to-electrical assembly configured to receive a first opticalsignal containing a first data signal and a first supervisory signal andseparate the first optical signal into the first data signal and thefirst supervisory signal; and an electrical-to-optical assemblyconfigured to receive a second data signal and a second supervisorysignal, and generate a second optical signal containing the second datasignal and the second supervisory signal.
 2. The optical transceiver ofclaim 1, wherein the first supervisory signal is superimposed onto thefirst data signal to form the first optical signal, and the secondsupervisory signal is superimposed onto the second data signal to formthe second optical signal.
 3. The optical transceiver of claim 2,wherein the frequency of the first data signal and the frequency of thesecond data signal are at least 1 GHz.
 4. The optical transceiver ofclaim 3, wherein the frequency of the first supervisory signal and thefrequency of the second supervisory signal are about 10 kHz.
 5. Theoptical transceiver of claim 1, wherein the optical-to-electricalassembly includes a limit amplifier for extracting the first data signaland serially-connected bandpass filter and operational amplifier forextracting the first supervisory signal.
 6. The optical transceiver ofclaim 5, wherein the optical-to-electrical assembly further includesserially-connected optical receiver unit and trans-impedance amplifierfor receiving an optical signal and converting the optical signal to avoltage signal that is input to the limit amplifier and the bandpassfilter.
 7. The optical transceiver of claim 1, wherein theelectrical-to-optical assembly includes a laser driver for superimposinga signal containing the second supervisory signal onto the second datasignal.
 8. The optical transceiver of claim 7, wherein theelectrical-to-optical assembly further includes an oscillator formodulating the second supervisory signal to a higher frequency signal,and the higher frequency signal is superimposed onto the second datasignal by the laser driver.
 9. The optical transceiver of claim 8,wherein the frequency of the second supervisory signal is about 10 kHz,and the frequency of the higher frequency signal is about 1 MHz, and thefrequency of the second data signal is about 1 GHz.
 10. A smallform-factor pluggable transceiver comprising the optical transceiver ofclaim
 1. 11. A method of transmitting a supervisory signal between afirst and second node of an optical communication network, comprisingthe steps of: receiving a supervisory signal; combining the supervisorysignal with a data signal having a frequency of at least 1 GHz;converting the combined signal to an optical signal; and transmittingthe optical signal containing the supervisory signal from the first nodeto the second node.
 12. The method of claim 11, wherein the step ofcombining includes the step of modulating the supervisory signal to ahigher frequency signal, wherein the higher frequency signal containingthe supervisory signal is combined with the data signal.
 13. The methodof claim 12, wherein the frequency of the supervisory signal is about 10kHz, and the frequency of the higher frequency signal is adjustable andis substantially separated from the frequency of the supervisory signaland the frequency of the data signal.
 14. The method of claim 13,wherein the frequency of the higher frequency signal is about 1 MHz. 15.The method of claim 11, wherein the step of combining includes the stepof superimposing the supervisory signal onto the data signal.
 16. Amethod of extracting a supervisory signal from a combined signalreceived from a node of an optical communication network, comprising thesteps of: receiving a combined signal having the supervisory signal anda data signal having a frequency of at least 1 GHz; extracting the datasignal; and extracting the supervisory signal.
 17. The method of claim16, wherein the step of extracting the supervisory signal includesfiltering low and high frequency components of the combined signal,wherein the supervisory signal is extracted from the filtered signal.18. The method of claim 17, wherein the supervisory signal is extractedfrom the filtered signal using an operational amplifier.
 19. The methodof claim 17, wherein the frequency of the supervisory signal is about 10kHz.
 20. The method of claim 19, wherein the frequency of the filteredsignal is about 1 MHz.